Soybean, Glycine max (L.) Merril, is a major economic crop worldwide and is a primary source of vegetable oil and protein (Sinclair and Backman, Compendium of Soybean Diseases, 3rd Ed. APS Press, St. Paul, Minn., p. 106. (1989). Growing demand for low cholesterol and high fiber diets has increased soybean's importance as a health food.
Soybean varieties grown in the United States have a narrow genetic base. Six introductions, ‘Mandarin,’ ‘Manchu,’ ‘Mandarin’ (Ottawa), ‘Richland,’ ‘AK’ (Harrow), and ‘Mukden,’ contributed nearly 70% of the germplasm represented in 136 cultivar releases. To date, modern day cultivars can be traced back from these six soybean strains from China. In a study conducted by Cox et al., Crop Sci. 25:529-532 (1988), the soybean germplasm is comprised of 90% adapted materials, 9% un-adapted, and only 1% from exotic species. The genetic base of cultivated soybean could be widened through exotic species. In addition, exotic species may possess such key traits as disease, stress, and insect resistance.
The availability of a specific micronutrient, such as iron (Fe), is often related to soil characteristics. Soil pH has a major impact on the availability of Fe. Iron deficiency has been a common, serious, and yield-limiting problem for soybean production in some parts of the United States.
Iron is one of the necessary micronutrients for soybean plant growth and development. It is also needed for the development of chlorophyll. Iron is involved in energy transfer, plant respiration, and plant metabolism. It is also a constituent of certain enzymes and proteins in plants. Iron is necessary for soybean root nodule formation and plays a role in N-fixation; thus, low levels of Fe can lead to reduced nitrogen content and poor yield.
When iron is limited, soybean plants can develop iron deficiency chlorosis (IDC). Soybean IDC is the result of a complex interaction among many factors including soil chemistry, environmental conditions, and soybean physiology and genetics. The most common IDC symptom is interveinal chlorosis in which leaf tissue of newly developed soybean leaves turn yellow, while the veins remain green. The leaves may develop necrotic spots that eventually coalesce and fall off the plant. Iron deficiency symptoms are similar to that of Manganese (Mn) deficiency; therefore, only soil and tissue analysis can distinguish the two micronutrient deficiencies.
Severe yield reductions due to IDC have been reported throughout the North-Central U.S with losses estimated to be around $120 million annually. In some instances, yield loss can be greater than 50%. Typically, soybean IDC symptoms occur between the first and third trifoliate stage, so under less-severe iron deficiency conditions, symptoms may improve later in the season.
Soybean plants grown in calcareous soils with a pH of greater than 7.4 or in heavy, poorly drained, and compacted soils may exhibit IDC symptoms due to insufficient iron uptake. However, soil pH is not a good indicator and does not correlate very well with IDC. Symptoms are highly variable between years and varieties and depend on other soil factors and weather conditions.
There is, however, a direct relationship between IDC incidence and concentrations of calcium carbonate and soluble salts. Iron uptake is adversely impacted by high concentrations of phosphorous (P), manganese (Mn), and zinc (Zn). Moreover, high levels of calcium (Ca) in the soil cause Fe molecules to bind tightly to the soil particles, making them unavailable for uptake. Therefore it is important to monitor the levels of calcium carbonate and soluble salts in the soil. Sandy soils with low organic matter may also lead to a greater incidence of IDC symptoms.
Weather also plays a role in IDC symptoms. Cool soil temperature and wet weather, combined with marginal levels of available Fe in the soil can increase IDC symptoms.
Soybean producers have sought to develop plants tolerant to low iron growth conditions (thus not exhibiting IDC) as a cost-effective alternative or supplement to standard foliar, soil and/or seed treatments (e.g., Hintz et al. (1987) “Population development for the selection of high-yielding soybean cultivars with resistance to iron deficiency chlorosis,” Crop Sci. 28:369-370). Studies also suggest that cultivar selection is more reliable and universally applicable than foliar sprays or iron seed treatment methods, though environmental and cultivar selection methods can also be used effectively in combination (Goos and Johnson (2000) “A Comparison of Three Methods for Reducing Iron-Deficiency Chlorosis in Soybean” Agronomy Journal 92:1135-1139; and Goos and Johnson “Seed Treatment, Seeding Rate, and Cultivar Effects on Iron Deficiency Chlorosis of Soybean” Journal of Plant Nutrition 24 (8) 1255-1268).
Soybean cultivar improvement for IDC tolerance can be performed using classical breeding methods, or, more preferably, using marker assisted selection (MAS). Genetic markers for low iron growth condition tolerance/susceptibility have been identified (e.g., Lin et al. (2000) “Molecular characterization of iron deficiency chlorosis in soybean” Journal of Plant Nutrition 23:1929-1939). Recent work suggests that marker assisted selection is particularly beneficial when selecting plants because the strength of environmental effects on chlorosis expression impedes progress in improving tolerance. See also, Charlson et al., “Associating SSR Markers with Soybean Resistance to Iron Chlorosis,” Journal of Plant Nutrition, vol. 26, nos. 10 & 11; 2267-2276 (2003). U.S. Pat. Nos. 7,977,533 and 7,582,806 disclose genetic loci associated with iron deficiency tolerance in soybean.
There is a need in the art of plant breeding to identify additional markers linked to genomic regions associated with tolerance to low iron growth conditions (e.g., IDC tolerance) in soybean. There is, in particular, a need for numerous markers that are closely associated with low iron growth condition tolerance in soybean that permit introgression of such regions in the absence of extraneous linked DNA from the source germplasm containing the regions. Additionally, there is a need for rapid, cost-efficient methods to assay the absence or presence of IDC tolerance loci in soybean.